Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to integrated circuit fabrication, and more particularly to a scanning method for use in a defect inspection system.
2. Description of the Background Art
During the process of integrated circuit fabrication, masks and wafers at various stages are inspected for defects. The inspection process consists typically of sequentially scanning areas of the mask, or wafer, with a beam of light while taking accurate measurements to detect if a defect exists at each location scanned. The beam is retained at any particular spot on the item under inspection for a period of time related to the accuracy of the measurement required, and the measurement results are compared against a threshold value to determine if the location contains a defect. The measurement accuracy, and thereby the time required to perform the measurement, depends largely on the number of false alarms which are permissible per unit of area when performing the test.
Properties of a defect inspection system include such important metrics as throughput, capture-rate, and false-alarm rate for a defect of a given size. The throughput is the number of units (i.e. mask blanks, or wafer blanks) that can be inspected per unit of time at a specified capture-rate and false-alarm-rate. Throughput is dependent on scanning speed, which is defined as the time to scan a unit area of the mask blank. False-alarm-rate is the probability of a non-defective area being considered defective during the test scan. The capture-rate is the probability that a defect of a given size will be detected by the test scan. Both false-alarm-rate and capture-rate depend on quality of the measurement. Within an inspection system that employs single level scanning, the quality of the measurement can also be considered as the signal-to-noise ratio that should be achieved so that a measurement threshold value can properly distinguish a defect.
To more fully exemplify defect scanning, the scanning of (Extreme UltraViolet Lithography) EUVL mask blanks is described. One form of defect inspection is to inspect reflective EUVL mask blanks using an at-wavelength inspection tool. Reflective masks contain multiple reflective layers whose spacing relates to the intended wavelength to be reflected. Reflective masks therefore can only be accurately tested for subsurface defects if the test is performed with an at-wavelength beam. One form of testing EUVL mask blanks involves scanning the area of the mask with a small diameter EUV beam (approximately 1.7xc3x975 xcexcm) and measuring changes in reflected intensity (bright field detection), scatter intensity (dark field detection), and/or the photoemissive current. In order for small defects to be detected, the size of the incident beam is very small, while the measurement itself must be taken over a large enough interval to assure accuracy. An inspection station used for this testing process typically comprises a mask blank held on a moveable stage within a vacuum chamber operating at about 10xe2x88x926 Torr. A small spot of EUV light is created by demagnifying a beam from an illuminating pinhole through a pair of Kirkpatrict-Baez (KB) mirrors. The beam is focused on the sample at approximately 9xc2x0 off-normal. A channeltron electron multiplier may be used for a bright field detector, while a microchannel plate with a reflective beam aperture for the bright field may be used as a dark field detector. To attain accurate defect inspection by this method the mask is generally inspected as a set of pixels, each about 3xc3x975 xcexcm, wherein each pixel is tested for approximately 50 mS.
A mathematical description of a defect inspection device as a shot-noise limited system can provide further explanations of capture-rate and false-alarm-rate for a defect of a certain size. A shot-noise system is one in which the dominant noise source is a shot-noise due to the finite number of photons detected by the detector. In a shot-noise limited system the distribution of signal can be represented by a Gaussian distribution with a mean m and standard deviation "sgr".                               f          ⁡                      (            x            )                          =                              (                          1                                                                    2                    ⁢                                          xe2x80x83                                        ⁢                    π                                                  xc3x97                σ                                      )                    ⁢                      e                                          -                                                      (                                          x                      -                      m                                        )                                    2                                            /                              (                                  2                  ⁢                                      xe2x80x83                                    ⁢                                      σ                    2                                                  )                                                                        (        1        )            
The signal from the clear region, or the region where there is no defect, can therefore be represented by a Gaussian distribution centered at m2, furthermore a signal from a 100 nm defect provides another Gaussian distribution centered at m1. Assuming simple area scaling of signal strength from a defect, and the spot size of 1 um, m1=0.99 m2. Simplified for m2=1, then m1=0.99. The standard deviation "sgr", is determined by the number of photons detected per pixel, wherein the area of each pixel is assumed to be the same as the spot size. The following relationship then holds for "sgr".
"sgr"=1/Ndxe2x80x83xe2x80x83(2)
The value Nd is the number of photons detected per pixel. Based on these assumptions, the capture-rate is determined by the probability of a 100 nm defect generating a signal smaller than threshold value s. A defect herein is assumed to cause a reduction of bright field signal such that the measured signal is smaller than the threshold.
capture_rate=P(x less than s;m1,"sgr")xe2x80x83xe2x80x83(3)
False-alarm rate is the probability of the clear region giving a signal smaller than the threshold value.
false_alarm_rate=P(x less than s;m2,"sgr")xe2x80x83xe2x80x83(4)
As described above, the false-alarm-rate is the probability P of the signal (or pixel value) being lower than a threshold value s, when the distribution is characterized by mean m2 and standard deviation "sgr". FIG. 1 shows a distribution corresponding to m1, and a second distribution corresponding to m2, both of which are shown in relation to the threshold value s.
Using the error function erf(x), the capture and false-alarm rates can be cast into a form which is an integration of the Gaussian distribution. The error function erf(x) is given by:                               erf          ⁡                      (            x            )                          =                              2            xc3x97                                          ∫                0                x                            ⁢                              e                                  -                                      t                    2                                                                                            π                                              (        5        )                                          P          ⁡                      (                                                            x                   less than                   s                                ;                                  m                  1                                            ,              σ                        )                          =                              ∫            s            ∞                    ⁢                      f            ⁡                          (              t              )                                                          (        6        )            
where f(t) is the normalized Gaussian distribution. The integration variable is changed and the following is therefore derived:                               P          ⁡                      (                                                            x                   less than                   s                                ;                                  m                  1                                            ,              σ                        )                          =                              1            +                          erf              ⁡                              (                                                      s                    -                                          m                      1                                                                                                  2                      ⁢                                              xe2x80x83                                            ⁢                      σ                                                                      )                                              2                                    (        7        )            
The capture-rate and false-alarm-rate can therefore be expressed as:                               capture_rate          ⁢                      (                          s              ,              σ                        )                          =                              1            +                          erf              ⁡                              (                                                      s                    -                    0.99                                                                              2                                        xc3x97                    σ                                                  )                                              2                                    (        8        )                                          false_alarm          ⁢          _rate          ⁢                      (                          s              ,              σ                        )                          =                              1            +                          erf              ⁡                              (                                                      s                    -                    1                                                                              2                                        xc3x97                    σ                                                  )                                              2                                    (        9        )            
The scanning time required per unit area is given by:                                                                         T                /                A                            =                              xe2x80x83                            ⁢                                                (                                      dwell time per pixel                                    )                                xc3x97                                  (                                      no. of pixels per unit area                                    )                                                                                                        =                              xe2x80x83                            ⁢                                                                    Nd                    /                                          F                      0                                                        xc3x97                  Np                                =                                  Np                  /                                      (                                                                  F                        0                                            xc3x97                                              σ                        2                                                              )                                                                                                          (        10        )            
where F0 is the total number of photons focused onto the 1 xcexcm spot per unit time, while Np is the total number if pixels per unit area (Np=108 per cm2 for 1 xcexcm spot size). Therefore, for any given spot size, minimum capture-rate, and maximum false-alarm-rate, the scanning time is mainly determined by the standard deviation of the Gaussian distribution of the signal.
As an example, when the signal to noise ratio is at 2 (i.e. "sgr"=0.5%) and the threshold is set at 0.99, the capture-rate is 50% and the false-alarm-rate is 2.28% with the false-alarm count being 2.28e6 per cm2. The scanning time for F0=1.4e8 is therefore approximately 8 hours per cm2. The value F0=1.4e8 is one that has been achieved using a 10xc3x97 Schwarzchild with a 100 xcexcm2 aperture with 100 xcexcm exit slit with a grating. Using white light with a 20xc3x97 Schwarzchild, the number increases about 4*50=200. The factor of 50 results from the expected flux increase anticipated from using the white light approach. Therefore it appears that at least F0=2.8e10 can be achieved.
As a further example, if we wish to obtain a capture-rate of 90% and a false-alarm-rate less than 1 count per cm2, the value of s would need to be set for s=0.9916 and "sgr"=0.15%, with a scanning time of 95 hours per cm2 for F0=2.8e10. Since the false-alarm-rate is so sensitive to "sgr" while the scanning time is only the inverse square of "sgr", the false-alarm-rate can be lowered without sacrificing too much of the scanning time.
Inspection of mask blanks and wafers in the above described single stage process has been shown above to often be a slow, and therefore costly, process.
Accordingly a new method is needed that will speed the process of inspecting blank masks and wafers for defects. The present invention satisfies that need, as well as others, and overcomes deficiencies in current inspection techniques.
The present invention is a multi-level scanning method that can increase throughput when used with defect inspection systems, such as the one described for inspecting reflective EUVL mask blanks. The use of multiple scanning stages under many circumstances, although counterintuitive, can provide additional throughput in relation to single stage scanning.
Conventional defect scanning techniques teach the use of a single scan in which measurements are taken at a signal to noise ratio adequate for determining which pixels are clear of defects. The measurements are taken to a level of measurement accuracy that will generate a rate of false-alarms below a given threshold. In a single scan, the number of false alarms resulting by the end of the scan should be less than the desired false-alarm-rate upon which the signal to noise of the measurement was based for the device being tested. In contrast, the present invention provides a multiple scanning approach that can increase throughput. By way of example, and not of limitation, a first stage of scanning employs a lower signal to noise ratio (more false alarm errors), such that the entire area can be inspected in a fraction of the time required to reach the desired false-alarm-rate in a single stage of scanning. Subsequent stages of defect inspection check only those areas failing the defect threshold comparison in the previous inspection stage, and the positionally uncertain areas around these probable defects, with a measurement that provides a higher signal to noise ratio corresponding to a lower false alarm rate. In the final stage of subsequent scanning, the areas which have been found as defects within the prior stage are tested at the final signal to noise ratio corresponding to a final allowed false alarm rate. The benefit of the method depends on the comparative times required to reach the various false-alarm-thresholds, the repeatability of the scanning system, and the speed of the scanning system to seek defect areas. This multi-level inspection method provides the same results as single-level testing, yet it can in many cases be performed in less time than that required for the single-level measurement. In other words, since progressively smaller regions are scanned within each stage, the overall throughput of defect inspection can be increased in situations where a favorable relationship exists between the inspection variables.
An object of the invention is to increase the throughput of defect inspection systems.
Another object of the invention is to retain precise control of the false-alarm-rate allowed by the defect inspection process.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.